Lecture 4 • Eclipses • Geocentric vs Heliocentric Theory •The Nature of Scientific Theories

The Planets

Prof. Geoff Marcy © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Crescent Moon and Venus visible sunday night .

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Review of Last Lecture

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Motion of the Night Sky as hours pass

1. From Berkeley 2. From the Equator 3. From the North Pole © 2005 Pearson Education Inc., publishing as Addison-Wesley

Berkeley Hills © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Moon: Going through phases each 29.5 Days (one orbital period) © 2005 Pearson Education Inc., publishing as Addison-Wesley

If you stand on the Moon, does the Earth go through “phases” ? Yes !

When Earthlings see a crescent moon, what is the phase of the Earth, as seen from the Moon?

a. New b. Crescent c. Quarter d. Gibbous © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Section

1

Eclipses: Solar and Lunar

• What do they look like?

• Why do they happen?

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Eclipses

Solar Eclipse: Moon blocks the Sun

• Sun is behind the moon: Only occurs at new moon You are in the Moon’s shadow.

within umbra: total solar eclipse within penumbra: partial solar eclipse © 2005 Pearson Education Inc., publishing as Addison-Wesley

2002 total Solar Eclipse Ceduna, Australia Dec 4, 2002 © 2005 Pearson Education Inc., publishing as Addison-Wesley

Solar Eclipse

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Solar Eclipse 1999 Aug 11 from the Russian Mir Space Station © 2005 Pearson Education Inc., publishing as Addison-Wesley

Eclipses

• The Earth & Moon cast shadows.

• When either passes through the other’s shadow, we have an

eclipse

.

• Why don’t we have an eclipse every full & new Moon?

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Sun

Total Solar Eclipse Lusaka, Zambia 2001 © 2005 Pearson Education Inc., publishing as Addison-Wesley

Solar Eclipse in India 24 October 1995 By: Solar Physicsts Wendy Carlos and Fred Espenak India © 2005 Pearson Education Inc., publishing as Addison-Wesley

Solar Eclipse

May 20 2012 Just after Finals Last Spring Sproul Plaza

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Solar Eclipse Predictions

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Total Solar Eclipse: Aug 21, 2017 © 2005 Pearson Education Inc., publishing as Addison-Wesley

Solar Eclipse: Aug 21, 2017

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Lunar Eclipses

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Section

2 A Model of the Solar System: Geocentric vs Heliocentric © 2005 Pearson Education Inc., publishing as Addison-Wesley

We see only one side of the Moon © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why we always see the same face of the Moon

Rotation period = orbital period Not Correct Model Correct Model Moon Moon Earth Earth © 2005 Pearson Education Inc., publishing as Addison-Wesley

Build a

model

, i.e. conceptual

theory

, of the Solar System

Conceptual models must explain all the motions of the planets: i.e., the “data”. . .

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Planets: Moving relative to the Stars

Obvious to the eye • Mercury – Difficult to see; Always angularly close to Sun • Venus – Very bright. Always near Sun— morning or evening “star” • Mars – Noticeably orange. Usually moves west-to-east – Sometimes backwards “retrograde” !

• Jupiter – Very bright. Moves west-to-east against stars.

• Saturn – Moderately bright. Moves more slowly west-to-east.

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Motion of the Planets

Relative to the Earth

during several years Earth The Sun and planets seem to orbit the earth during months and years .

Here: Sun Mercury Venus Mars © 2005 Pearson Education Inc., publishing as Addison-Wesley

The Motion of Planets • The Planets normally move West to East against the background stars.

• Why do planets sometimes seem to move backwards relative to the stars?

• Greeks concluded that the planets orbit the Earth. Why did smart people conclude this?

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Observed Motion of Mars: Normal & “Retrograde”

”

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To Explain Retrograde Motion

Two Models:

Geocentric Heliocentric © 2005 Pearson Education Inc., publishing as Addison-Wesley

Ptolemy’s

Geocentric

Model

• Earth is at center • Sun orbits Earth • Planets orbit on small circles whose centers orbit the Earth on larger circles (The small circles are called

epicycles

) © 2005 Pearson Education Inc., publishing as Addison-Wesley

In Heliocentric “model”

Retrograde Motion is a Natural Consequence • Planets usually appear to move eastward relative to the stars.

• But as we pass by them, planets seem to move west relative to the stars. • Only noticeable over many nights; on a single night, a planet rises in east and sets in west… © 2005 Pearson Education Inc., publishing as Addison-Wesley

Two Theories: : Earth-Centered Theory Sun-Centered Theory Which theory Seems “Best” ?

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Choosing the Best Model: Explaining Retrograde Motion • Natural result of Heliocentric Model • Difficult to explain if Earth is at center The Best “Model” or “Theory” explains various data and phenomena with the fewest assumptions.

“Occam’s Razor “: Choose the simplest model that explains all the data.

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3D “model” of the Solar System © 2005 Pearson Education Inc., publishing as Addison-Wesley

Why did the Greeks reject the theory that the Earth orbits the Sun?

• It ran contrary to their common sense: Every day, the sun, moon, and stars rotates around us. So, we “must be” at the center . . .

• If the Earth rotated, then there should be a “great wind” as we moved through the air.

• Greeks knew that we should see stellar “parallax” if we orbited the Sun – but they could not detect it. © 2005 Pearson Education Inc., publishing as Addison-Wesley

Test the Theory that Earth orbits the Sun:

Parallax:

Apparent shift of a star’s position due to the Earth’s orbiting of the Sun.

Greeks didn’t detect parallax !

The nearest stars are much farther away than the Greeks thought.

The parallax angles of the stars are so small, that you need a telescope to observe them.

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Two Possible reasons why stellar parallax was not detected:

1. Stars are so far away that stellar parallax is too small for naked eye to notice.

2. Earth does not orbit Sun; it is the center of the universe.

Debate about theory: Earth-centered vs. Sun-centered Planetary System.

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Section

3 Scientific Theory: What is a good “Theory” ?

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Scientific Thinking

• It is a natural part of human curiosity: Search for understanding and truths that explain many facts.

• We draw conclusions based on our experiences.

• Progress is made through “trial and error.” Hypothesize. Then test your hypothesis.

Eating pasta makes me get fat . . .

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Nicolaus Copernicus

(1473-1543) He thought Ptolemy’s model was contrived.

Artificial construction; Too complicated.

De Revolutionibus Orbium Coelestium

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Copernicus’

Heliocentric

Model

• Sun is at center • Earth orbits like any other planet • Inferior planet orbits are smaller • Retrograde motion occurs when we “lap” Mars and the other superior planets © 2005 Pearson Education Inc., publishing as Addison-Wesley

Johannes Kepler

(1571-1630) • Greatest theorist of his day • Imagined planets on “heavenly spheres” © 2005 Pearson Education Inc., publishing as Addison-Wesley

Kepler’s Laws

1. Each planet’s orbit around the Sun is an

ellipse,

with the Sun at one focus.

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Eccentricity of an Ellipse © 2005 Pearson Education Inc., publishing as Addison-Wesley

Kepler’s 2nd Law A planet moves along its orbit with a speed that changes in such a way that a line from the planet to the Sun sweeps out equal areas in equal intervals of time.

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Kepler’s 3rd Law

The cube of a planet’s average distance from the Sun is equal to the square of its orbital period. (Use units of years and AUs.) a 3 = P 2 © 2005 Pearson Education Inc., publishing as Addison-Wesley

Curiosity Landing on Mars 5 August 2012